Field of the Invention
The present invention relates to a battery type screw driver.
Related Background Art
A conventional AC/battery screw driver is provided with only a main switch without an auxiliary switch. Hence, a motor is actuated during operations other than an actual fastening process, which include an operation for confirming a screw fitting location and an operation for fitting of the screw to a bit. Consequently, there arise the following problems. (1) A loss in energy is caused especially when the battery serves as a motive power supply, resulting in a reduction inwork done per charging. (2) The motor and gear sounds are continuously generated during the operation to cause noises.
Japanese Patent Laid-Open Publication No. 61-76281 discloses a screw driver which can obviate the defects described above.
In the screw driver, the motor is rotated when the screw driver is pushed against a screw in a state of depressing the main switch. For this purpose, as illustrated in FIG. 17, the screw driver includes a microswitch as an auxiliary switch 1. This auxiliary switch 1 is disposed in series with the main switch in a driving circuit of the motor. The auxiliary switch 1 does not function simply by applying the bit 2 to the screw 3 (FIG. 17A). When the bit 2 is pushed against the screw 3, a driving shaft 4, front cover 5a, and a load detection unit 5 are all together moved backwards by the reaction thereof. The auxiliary switch is thereby turned on (FIG. 17B). Then, the motor is actuated for the first time, thus rotating the bit 2 through the driving shaft 4.
The conventional screw driver described above is, however, provided with the ring-like load detection unit 5. The load detection unit 5 slides in accordance with a sliding motion of the driving shaft 4. The auxiliary switch 1 is depressed by a projection 6 formed on the unit 5. The projection 6 exists in a position deviated largely from the driving shaft 4. There is a possibility that the operation of the auxiliary switch 1 is inaccurate. Further, the auxiliary switch 1 has to be provided in a gear box G behind the driving shaft 4. It is troublesome to install the auxiliary switch 1, and the auxiliary switch 1 is apt to be out of order. Besides, if grease in the gear box permeates the auxiliary switch 1, electrical trouble could result.